204 HENBY LAURENS 



In light it is thought that the constant illumination or stimula- 

 tion of the retinae starts impulses by certain photochemical 

 changes in the retinae, the end effects of which are opposite to 

 those of direct stimulation, and in this way bring about the 

 secondary reaction of contraction. In darkness the same kind 

 of thing is supposed to take place, long continued absence of 

 light producing impulses by chemical changes, the end effects 

 of which result in the expansion of the melanophores. That the 

 secondary responses do not take place in the eyeless larvae shows 

 that the seat of the causes of them must be sought in the retinae. 

 The eyes through the nervous system cause the melanophores 

 to secondarily contract in light and to expand in darkness. The 

 results obtained with the larvae of Amblystoma tigrinum are 

 thus in complete agreement with those earlier obtained with the 

 larvae of A. punctatum and A. opacum, and do not agree with 

 those of Babak. 



SUMMARY 



1. The reactions of the melanophores of the larvae of Ambly- 

 stoma tigrinum are exactly like those earlier obtained with the 

 larvae of A. punctatum and A. opacum. In light the melano- 

 phores expand (figs. 1 and 2) and in darkness they contract 

 (figs. 3 and 4), in both seeing and eyeless larvae. The melan- 

 ophores of seeing larvae that have been kept for some time 

 (four days or more) in bright diffuse light over an indifferent 

 bottom are, however, partly contracted (| to j expansion) (fig. 

 5) while the melanophores of seeing larvae that have been kept 

 for some time in darkness (more than five days) are expanded 

 (f to I expansion) (fig. 6), thus showing, under long continued 

 illumination and darkness, what has been called a secondary 

 reaction. 



